Boldly Going Part 28
In 2009,
Space Station Enterprise celebrated the end of its second decade in space. The milestone provided a convenient marker for evaluating how NASA’s human spaceflight program had evolved since the station’s launch. At the orbital outpost, NASA had accumulated tens of thousands of crew-days in space in 14 years of permanent occupation, along with various visiting Shuttle crews. After the explosive growth of the late 1990s, the station’s core configuration had remained relatively constant. Instead, the main changes had come with the concentration of space assets like
Galileo and the Hubble Space Telescope in orbits near
Enterprise. With this concentration and a growing focus on sustained scientific capabilities at the station and lunar outposts, flights of the Space Shuttle to any destination other than
Enterprise grew increasingly rare. The Space Shuttle had achieved its role as a space truck to serve a constellation of space vehicles, but its independent operational capabilities were increasingly unnecessary. Instead, these capabilities were overshadowed by the aging of the vehicles themselves and growing frustration with the Shuttle’s maintenance requirements. Rockwell’s OPAMs for the Shuttle-C heavy lifter and Lockheed Martin’s reusable engine pods for Atlas-III and Shuttle LRBs, coming off their own first decade in service, provided a notable point of comparison between the “first generation” Shuttle hardware and the “second generation” spawned by Shuttle-C.
Similar struggles were experienced in the lunar program as the Minerva office moved from early exploration and base development into routine flights to consolidate operations at the three Minerva “Class-C” base sites. With Shackleton Base receiving its sixth crew in 2009, the one-way flights of the descent stages became more and more of a problem. The cost of providing a new
Conestoga lander for each crew or cargo logistics flight began to be a significant contributor to the cost of supplying and occupying Shackleton. The Shuttle-C Earth Departure Stage was used for some DoD Atlas III Heavy flights, and the Kepler-L lunar crew capsule shared many of its systems with the fleet of Earth-orbital capsules used for
Enterprise lifeboats and independent European crew launches.
Conestoga had no such cost savings. The high resulting cost was a restriction on any plans to uprate any of the other Minerva sites to a Class C outpost for intermittent or permanent crew occupation. Indeed, NASA was increasingly considering their future plans beyond cislunar space entirely, which would require costs of the existing programs to be minimized and allow for new development. Discontinuing permanent lunar occupation or ending operations at
Enterprise was politically unpopular. Thus, as Mars loomed larger in NASA’s plans, the agency would have to find ways to cut costs in the aging Space Shuttle and
Conestoga programs.
The focus on replacing the first generation Space Shuttle and converting
Conestoga to a reusable operational strategy reflected the change in NASA’s operations since the two vehicle’s respective births. Though the Space Shuttle had been the workhorse of the American program for nearly thirty years, its roles were being increasingly rendered unneeded by other vehicles. Independent flights with Spacelab or Spacehab to orbits other than that of
Enterprise had largely stopped, and Shuttle-C and Atlas III had taken over the role of the American pre-eminent heavy lift vehicles for large probes and lunar missions. Just as the Space Shuttle’s LRBs had increased its payload above that originally envisioned in the 1970s, the assembly of
Enterprise was completed largely using the existing SRBs. For a few years, the augmented capability had been consumed with ethanol-LOX propellants to refuel the
Galileo tug/station for its task of moving Hubble to near
Enterprise’s orbit. Since then, it had largely been employed for whatever odd tasks NASA could find for bulk liquids and solids on the station or simply gone unused in the name of increased redundancy and safety. The higher performance demanded from the full-throttle SSME-69 engines on the orbiters made for higher maintenance requirements relative to the otherwise identical SSME-69 and SSME-35 engines on Shuttle-C and the LRBs respectively. For the latter, as part of an Atlas-III rapid response demonstration, Lockheed Martin and Rocketdyne had recently demonstrated five full-duration firings of an Atlas-III core on a test stand in under a week. Further, the orbiter’s hypergolic-fueled APUs and OMS made for challenges in ground handling and spacecraft processing which the all-cryogen Shuttle-C OPAMs simply didn’t experience. While wearing “bunny suits” to clean and remove an OMS pod for servicing, crews at the Orbiter Processing Facilities couldn’t help but compare the “hangar queens” to the work-horse Shuttle-C Propulsion & Avionics Modules. With their electromechanical actuators and non-toxic ethalox thrusters, servicing a pair of OPAMs for a Minerva lunar mission took less time than servicing a single Shuttle Orbiter for a crew mission.
Spares were also a concern as the orbiter’s primary structures and secondary systems aged. No new orbiter components had been manufactured since the construction and delivery of OV-105
Endeavour in 1991. NASA was carefully monitoring wear and fatigue cracking of the orbiter’s primary structures as they husbanded the spare wings and tail collected from the butchering of OV-101 into
Space Station Enterprise. In contrast, Rockwell had delivered a total of five OPAMs in the last decade, culminating in the deliveries of OV-203
George Mueller and OV-204
Richard Byrd in 2002 and the refurbishment of the test vehicle OV-200, retroactively the
Pete Conrad, to reach near-flight status in 2003. None of the OPAMs was more than seven years out of a major overhaul, and Lockheed Martin was still intermittently producing batches of LRB/Atlas-III engine pods to keep the operational Air Force and NASA booster fleet over a dozen.
Once payloads reached the moon, the task of ferrying them down to the surface was another bottleneck. As Minerva had moved from sorties and base construction into regular operations,
Conestoga’s design advantages had transformed into limitations. While it lacked Shuttle’s direct comparison to the Shuttle-C and LRB, its faults were no less apparent to those who had to work on and fund its operation. Early in base construction,
Conestoga’s clever expendable descent stage, with its two Habitank wet-workshop hydrogen tanks, had been useful as every landing in the buildout of the site provided not just crew or cargo, but also free volume. However, by the fourth crew rotation, the number of LSAMs landed at Shackleton began to exceed the capability of the crews at the base to convert or even
attach to the base given the limited numbers of ports available on each Minerva Core Module. NASA was required to provide every crew and logistics mission with a new
Conestoga for a one-way flight to the moon, where a “boneyard” worth hundreds of millions of dollars began to collect just off the cleared and sintered landing pad. Each new arrival, once its cargo or crew were unloaded, was towed by mule rovers to a resting spot just beyond the berm protecting the base from flying debris. Once in line behind the berm, the precision engineered hardware was simply abandoned in place in case of a future need which seemed to grow increasingly remote. The
Conestoga’s greatest benefit - its adaptable expendable descent stage - had become its greatest downside. Even in the early 2000s, NASA had investigated long-term design improvements to let the lander be refilled and serviced in lunar or Earth orbit. This would allow a small set of landers to serve ongoing permanent lunar bases, and let the existing expendable ascent stages be replaced entirely. As Shackleton Base moved into regular nearly year-long crew missions,
Conestoga was rarely being called upon to push to the limits of its capacity. Thus, as with Shuttle, the cost of conducting
any flight to the lunar surface began to dominate over the benefit of any single marginal kilogram delivered per flight.
None of the issues with Shuttle or
Conestoga were new however, and options for fixing them had been in the works for the better part of a decade. Beyond the
Conestoga reuse plans, NASA had been entertaining options for a lighter, cheaper, and lower-maintenance “Shuttle-II”. In 2009, they were finally authorized to begin development of a full-on Shuttle successor and the implementation of the modifications to let
Conestoga become a reusable cislunar tug and reusable lander. Uncrewed spacecraft had proved their value over the previous decade with the successes of autonomous rendezvous on the
Galileo free-flyer/tug and
Conestoga cargo landers’ descent to the surface under internal control on cargo flights. For improved safety and reduced performance requirements, the plan was that the new design would not require crew on every mission. Instead, those missions requiring pure cargo performance could use the full capability, while crew missions would carry a crew compartment mounted in the payload bay. Another break came in the elimination of the primary external tank, the largest single disposable element of the Space Transportation System. Instead, the new vehicle’s design would consist of a 5-meter diameter fuselage, with a 12-meter-long payload bay sandwiched between an aft oxygen tank and a forward hydrogen tank. For cargo missions or Kepler lifeboat rotations, the vehicle’s designed payload of 14 metric tons would suffice for most
Enterprise logistics purposes. Though reduced from the theoretical limits of the existing Space Shuttles, the net delivered payload would be similar to what current
Space Station Enterprise logistics flights could accommodate due to center of gravity limits on abort landings. For crew flights, a cabin module would nestle into the bay, mounting to the same payload trunnions but leaving its exterior hull flush with and replacing the standard bay doors and thermal protection system. The crew module would provide spacious seating for up to 14 astronauts, offering cheaper and more capable crew transport to
Space Station Enterprise or other destinations. For safety, the surfaces of the pod inside the bay were to be covered with a single-use ablative heat shield, and the ends of the module would house six massive solid abort motors capable of blasting the crew cabin free of a disintegrating orbiter. This ensured that regardless of any issues on ascent or return, the crew of Shuttle-II had a way to get home. For launch, the new orbiter would be its own second stage. First stage boost would be a series-staged burn more like Atlas-III Heavy, with Shuttle-II hung from the side of a pair of existing Shuttle LRBs. The new vehicle would be able to tie into the same thrust mounts used by the existing Shuttle and Shuttle-C stack and the Atlas III Heavy side boosters. The design was frozen in 2011, and introduction into service was predicted for 2018.
At the same time NASA and the Rockwell-heritage areas of Boeing were working on the revised Shuttle-II orbiter design, Boeing’s McDonnell-heritage division was working on the plans for a reusable
Conestoga variant, dubbed
Diana. On-orbit servicing of the
Galileo ethanol/LOX propulsion system meant NASA was reasonably familiar with cryogenic propellant handling in space. The application of the concept to
Conestoga was reasonably straightforward. The main challenge came from the addition of hydrogen to the existing oxygen experience, and with it increased worries about thermal insulation. Efforts to minimize tank penetrations and increase propellant life in storage were aided by the design of the hydrogen system. The two large hydrogen tanks and single spherical “sump” tank, designed to enable a more effective Habitank, also helped minimize their surface area and number of tank penetrations. There were even proposals to use the airlock-to-tank interconnects for in-space inspection and servicing of tanks over extended design life, years down the line. This life was not unreasonable in NASA’s view, since the RL-10 engines used by the vehicle were already nominally capable of dozens of relights in space thanks to its spark-ignited, low-pressure, and low-temperature design burning non-coking hydrogen fuel. If the new Shuttle-II or another vehicle could serve as a tanker, then only moderate in-space servicing and a new crew cabin would be required to transform the LSAM descent stage into a single-stage reusable lunar lander. For the occasional deployment of larger structures, additional drop tanks would help preserve the vehicle’s capability for heavier payloads to the lunar surface. The challenge would depend on much the same factors that NASA’s plans for exploration beyond Earth orbit were hinging on: cost per flight of the new Shuttle-II and the presence of a structure in low Earth orbit for the servicing of smaller spacecraft and the assembly of larger ones headed to destinations like Mars.
Artwork by:
@nixonshead (
AEB Digital on Twitter)
